1. Technical Field
The present disclosure relates to station-keeping for synchronous satellites.
2. Description of Related Art
With reference to FIG. 1, a synchronous satellite 10 orbits the Earth 12 at a rate that matches the Earth's rate of revolution, so that the satellite 10 remains above a fixed point on the Earth 12. FIG. 1 illustrates the satellite 10 at two different points A, B along its orbit path 14. Synchronous satellites are also referred to as geostationary satellites, because they operate within a stationary orbit. Synchronous satellites are used for many applications including weather prediction and communications.
Various forces act on synchronous satellites to perturb their stationary orbits. Examples include the gravitational effects of the sun and the moon, the elliptical shape of the Earth and solar radiation pressure. To counter these forces, synchronous satellites are equipped with propulsion systems that are fired at intervals to maintain station in a desired orbit. For example, the satellite 10 illustrated in FIG. 1 includes a plurality of thrusters 16.
The process of maintaining station, also known as “station-keeping,” requires control of the drift, inclination and eccentricity of the satellite. With reference to FIG. 1, drift is the east-west position of the satellite 10 relative to a sub-satellite point on the Earth 12. Inclination is the north-south position of the satellite 10 relative to the Earth's equator. Eccentricity is the measure of the non-circularity of the satellite orbit 14, or the measure of the variation in the distance between the satellite 10 and the Earth 12 as the satellite 10 orbits the Earth 12. Typically, satellite positioning, and in some instances satellite orientation, is controlled from Earth. A control center monitors the satellite's trajectory and issues periodic commands to the satellite to correct orbit perturbations. Typically, orbit control is performed once every two weeks, and momentum dumping is performed every day or every other day.
Current satellites are either spin-stabilized or three-axis stabilized satellites. Spin-stabilized satellites use the gyroscopic effect of the satellite spinning to help maintain the satellite orbit. For certain applications, however, the size of the satellite militates in favor of a three-axis stabilization scheme. Some current three-axis stabilized satellites use separate sets of thrusters to control north-south and east-west motions. The thrusters may burn a chemical propellant or produce an ion discharge, for example, to produce thrust. Alternatively, the thrusters may comprise any apparatus configured to produce a velocity change in the satellite. The north-south thrusters produce the required north-south change in satellite velocity, or ΔV, to control orbit inclination. The east-west thrusters produce the required combined east-west ΔV to control drift and eccentricity. As the cost of satellite propulsion systems is directly related to the number of thrusters required for station keeping, it is advantageous to reduce the number of thrusters required for satellite propulsion and station keeping. Further, propulsion systems have limited life spans because of the limited supply of fuel onboard the satellite. Thus, it is also advantageous to reduce fuel consumption by onboard thrusters so as to extend the usable life of the satellite.